Imidazoles are an important class of compounds that are present in many drugs and biologically active molecules. In particular, nitroimidazoles have diverse biological and medical applications, ranging from use as, for example, antibacterial and antifungal agents to cancer tissue markers and chemotherapeutic agents. Nitroimidazoles also are used widely as hypoxia selective cytotoxic agents. See e.g., Rauth, A. M. et at Int. J. Radiation Oncology Biol. Phys. (1998) 42; 755-62. Hypoxia—a reduction of oxygen supply to tissue below physiological levels despite adequate perfusion of the tissue by blood—is a condition present in many solid tumors where the cells of the tumors are deficient in oxygen. In solid tumors, the hypoxia regions often surround areas of necrosis, which are areas of dead cells or tissues resulting from injury or disease that occur usually in a localized area of the body. Hypoxia also is observed in arthritic joints, inflammatory bowel disease and other conditions.
Because of its association with tumors, hypoxia is a very important diagnostic tool for tumor onset. Further, the design and development of drugs selectively toxic towards hypoxic cells are important because, for example: (1) tumor cells are severely more hypoxic than normal cells; (2) radiotherapy fails in hypoxic cells, and thus, as a treatment for solid tumors, since hypoxic cells are deficient in oxygen, which is a radiation sensitizer (i.e., an agent that makes cells and tissue susceptible to the effects of radiation); (3) bioreductive hypoxia selective cytotoxins (i.e., compounds that are specifically targeted towards, and toxic to, hypoxic tissues) kill hypoxic cells, which are resistant to most anticancer drug treatments; and (4) hypoxia in tumors often promotes a more malignant phenotype. See e.g., Brown, J. M. and W. R. Wilson, Nature Reviews (June 2004) 4: 437-47. Therefore, hypoxia-selective drugs and prodrugs that selectively release an active agent in hypoxic tissues are important to the treatment, prevention and control of tumors, such as solid tumors, and other conditions associated with hypoxia. As used herein, the terms “drug” and “active agent” refers to (A) articles recognized in the official United States Pharmacopoeia, the official Homoeopathic Pharmacopoeia of the United States, or any official National Formulary, or any supplement to any of them, (B) articles, including, e.g., chemical compounds or biologicals, intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease in man or other animals; and (C) articles intended to affect the structure or any function of the body of man or other animals, and (D) articles intended for use as a component of any article specified in clause (A), (B), or (C). As used herein, the term “biologicals” refers to medicinal preparations made from living organisms and their products, including, e.g., serums, vaccines, antigens, antitoxins, and the like. As used herein, the term “prodrug” refers to a reversibly modified agent that usually is inactive in itself, hut which releases an active agent when metabolized in the body. Often, endogenous enzymes act upon a prodrug thereby activating it upon reduction, and thus, triggering the hitherto latent functionality. A prodrug can (1) increase or decrease the metabolic stability of the active agent, (2) mask or reduce side effects and toxicity of an active agent, and (3) improve transport characteristics, solubility properties and the flavor of the active agent.
A specific type of prodrug is that known as a bioreactive drug or a bioreductively activated drug, which is an agent that is reduced in hypoxic regions (such as solid tumours) usually by one electron reduction (Denny, W. A. and W. R. Wilson, J. Med. Chem. (1986) 29(6): 879-87). The use of bioreactive drugs was investigated first by Lin et al in 1972 (J. Med. Chem. 15: 1247-52). If the reduced species of the bioreductive drug is the active form, then often the drug will have relatively few side effects. Ideally, the reduction will be reversible so that if the agent is reduced in an oxic region (i.e., an oxygen-containing region), it is oxidized back to a non-active form by molecular oxygen. As used herein, the term “reduced” or “reduction” refers to the loss of an oxygen atom from a compound with a gain of electrons by the compound, or a decrease in oxidation number (oxidation state) of a compound. As used herein, the term “oxidized” or “oxidation” refers to the addition of oxygen to a compound with a loss of electrons, or an increase in oxidation number (oxidation state) of a compound. As used herein, the phrase “oxidation number” or “oxidation state” refers to the number of electrons that must be added to or subtracted from an atom in a combined state to convert it to the elemental form, i.e., the form relating to, being or existing as an uncombined chemical element.
Bioreductively activated drugs are widely in demand for their specific and selective targeting of diseased tissues. Nitroimidazoles have been used as prodrugs for many years. The use of nitroimidazoles in anti-tumor therapy stems from the differences observed in the environment of, and the physiological concentration of molecular oxygen in, tumor versus normal tissues. Tumor cells reside in a much more acidic environment than normal cells, and as discussed previously, are severely more hypoxic than normal cells. In the context of tumor therapy and diagnosis of hypoxia, although 4- and 5-nitroimidazoles are studied, 2-nitroimidazoles are by far, the most extensively studied. See e.g., Shimamura, et al. Brit. J. Cancer (2003) 88: 307-13; Kasai et al. Bioorg. Med. Chem. (2001) 9: 453-64; Papadopoulou et al. Bioorg. Med Chem. (2004) 14: 1523-25. In particular, 2-nitroimidazoles have significantly higher reduction potentials (about −418 mV) as compared to unsubstituted nitrobenzene (about −486 mV) and hence, are known to be selective to hypoxic regions of tumors. The 2-nitroimidazoles, therefore, have been identified as one of the three structurally different classes of bioreactive drugs with selective toxicity towards hypoxic cells; the mitomycins and benzotriazine dioxides being the other two classes.
The 2-nitroimidazoles, mitomycins and benzotriazine dioxides, as bioreactive drugs, are activated selectively in the absence of oxygen to reactive intermediates that can damage deoxyribonucleic acid (DNA). In the case of 2-nitroimidazoles, several reactive reductive intermediates—namely, the nitroso and hydroxylamine intermediates, have been isolated and their interaction with DNA and cellular thiols have been assessed at the molecular level. See e.g., Cowan, D. S. M., et al. Br. J. Cancer (1994)70: 1067-74; Brezden, C. B., et al. Biochem. Pharmacol (1994) 48: 361-70. As used herein, the phrase “reductive intermediate” refers to an intermediate as defined below that has the ability to remove oxygen from a compound, or cause the compound to react with hydrogen or form a hydride, or to undergo an increase in the number of electrons. This has led to a realization that the target for cell killing by these bioreductive drugs may have a cellular redox component (i.e., a change in oxidation state component) and a direct DNA damage component, resulting in apoptotic cell death, which is one of the main types of programmed cell death that involves the deliberate relinquishment of life by a cell by a programmed sequence of events (i.e., “a cell suicide mechanism”). Derivatives of 2-nitroimidazole attached to DNA intercalating moieties have been synthesized to determine the degree to which targeting 2-nitroimidazoles to DNA affects the efficacy of 2-nitroimidazoles as radiosensitizers and hypoxic cell toxins. As used herein, the term “radiosensitizer” refers to a compound that enhances the effects of radiation, often by mimicking oxygen, which is a radiation sensitizer. For reviews on bioreductive therapy and synthesis of nitroimidazole prodrugs, see e.g., Jaffar et al. Adv. Drug Del. Rev. (2001) 53: 217-28; Naylor, M. A. and P. Thomson Mini Rev. Med. Chem. (2001) 1: 17-29; Hay et al. J. Med. Chem. (2003) 46: 5533-45.
Chemotherapy (meaning the use of chemical substances to treat or control disease) plays a vital role in killing cancerous cells of solid tumors because of the failure of radiotherapy to do the same. Derivatives of 2-nitroimidazole are used widely as prodrugs in chemotherapy owing to their hypoxia selectivity. For example, some widely used 2-nitroimidazole prodrugs are shown below: